Recent advances in geochronology, or rock dating, allow geologists to measure radioactive decay and assign very precise absolute ages to geological events. In another study, scientists found isotopic evidence of a sudden switch to primitive plate tectonics dating back about 3.8 billion years. The discovery suggests that a simple form of subduction had begun at least in one place on the planet by that time. And a tree-lined African lake has provided data on the Earth’s magnetic field for the past 150,000 years.
From the origin of life to mass extinctions, Earth has gone through incredible changes in its 4.6 billion years. With such a long history, how can researchers track what happened and when?
The system that many scientists have settled on is the International Geologic Time Scale (outlined here in the International Chronostratigraphic Chart ), which divides geologic time into five units. From longest to shortest and most precise, these units are eons, eras, epochs, periods, and ages.
Different stages of geologic time “are defined by visible changes in the fossil record,” says Jacqueline Gill, an associate professor of paleoecology and plant ecology at the University of Maine. Fossils are a useful tool in this dating work for several reasons. First, life has likely existed for about 90 percent of Earth’s existence, so Earth’s history parallels the history of life. Fossils are also useful because changes in the fossil record reflect changes in ecology, the relationship between living things and their environment. These changes in Earth’s ecology tend to reflect major events in the planet’s history, according to livescience.com.
One of the major moments in geological time was the transition from the Mesozoic Era to the Cenozoic Era about 65 million years ago. The change was caused by an asteroid impact that eventually killed off the non-avian dinosaurs.
International Chronostratigraphic Chart 2020. International Commission on Stratigraphy, stratigraphy.org
You don’t need a time machine to appreciate the magnitude of the changes between the Mesozoic and Cenozoic eras. Careful examination of a rock layer can be enough to help a researcher figure out its age. For example, the very impact that is thought to have ended the Mesozoic era and ushered in the Cenozoic era is marked by a layer containing unusually high levels of iridium, which is much more common in meteorites than in the Earth’s crust.
Other changes in mineral and elemental ratios can also serve as evidence of Earth’s turbulent history. For example, just over 5 million years ago, tectonic activity closed the Strait of Gibraltar, ultimately causing the Mediterranean Sea to shrink and its mineral composition to increase dramatically. This event occurred during the Messinian Period and marked the end of the Miocene Epoch and the beginning of the Pliocene Epoch, about 5.3 million years ago.
According to Karl Wegmann, an associate professor of geology at North Carolina State University, “salt and gypsum deposits are now seen in many countries surrounding the Mediterranean, and they are now exposed as rocks above present sea level” due to tectonic activity in the millions of years since this last happened.
Different layers of rock deposited over time in a slot canyon in Utah can be seen in Grand Staircase-Escalante National Monument. Shutterstock
Recent advances in geochronology, or rock dating, allow geologists to measure radioactive decay and assign very precise absolute ages to geological events. To do this, geochronologists calculate the age of rocks by comparing the proportions of certain isotopes, or elements that have different numbers of neutrons in their nuclei than normal. All of these approaches fit together like a puzzle to give researchers a common language for discussing the distant past.
The geologic time scale is a scientific tool, but it is also an artifact of history. Objective measurements of properties such as radioactive decay can tell researchers when rock layers formed, but it is up to scientists, often building on the work of their predecessors, to decide how to slice and dice the data into geologic time frames. One of the most contentious issues in chronostratigraphy is how to define our own time.
“The Holocene is kind of an arbitrary epoch,” Gill said. The Holocene epoch began about 12,000 years ago, when the Earth began to warm after the last ice age. But the end of that ice age, even though it coincided with the transition to a new epoch, is no more geologically significant than the ends of ice ages before it, Gill said.
Even today, scientists are still naming new time windows, including the Chibani Age, named for the Japanese prefecture where the sediments defining its age were found. Many scientists and others argue that the recent human impact on the planet merits declaring a new epoch, the Anthropocene, while others say the Capitalocene more accurately captures the social systems that have shaped the planet so much since the Industrial Revolution.
Can stones grow? To explain how and why, let’s first talk about another important question: what is a stone?
A rock is a collection of minerals. A mineral is a single crystal made up of elements such as silicon, oxygen, and carbon. A mineral becomes a rock when multiple crystals are present, regardless of whether the rock is made up of many different kinds of minerals or all of the same kind.
When water flows to the surface at hot springs like Mammoth Hot Springs in Yellowstone National Park (shown here), huge travertine formations form. Jim Pico, National Park Service
In caves, rocks grow because water running down the walls or dripping from the ceiling leaves minerals along the walls or floor of the cave. Cave rocks are usually made up of minerals such as calcite, aragonite, opal, chalcedony, and others. Rocks formed by this dripping water are called stalactites when they grow from the ceiling and stalagmites when they grow from the floor.
The so-called Puppet Theater in Carlsbad Caverns National Park is home to a vast collection of stalactites and straw columns. NPS Photo/Peter Jones
A type of rock called travertine grows in springs where water flows from underground to the surface. Large travertine formations are often found around hot springs because warm water contains more minerals than cold water. The famous Mammoth Hot Spring in Yellowstone National Park is only about 8,000 years old, but its travertine deposits are 239 feet (73 meters) thick and cover more than 1.5 square miles (4 square kilometers) of land.
Some rocks grow when dissolved metals precipitate out of seawater or fresh water. These so-called nodules, like the manganese nodules shown here, are found on the bottom of every ocean. Niels Brenke, CeNak
Water also contains dissolved metals that can “precipitate” out of seawater or freshwater to form rocks. These rocks are called nodules. Nodules of manganese, iron, copper, nickel, and cobalt are found on the bottom of every ocean. They form layer by layer, like a pearl. (Pearls are technically rocks, although they are produced by oysters.) Each layer makes the nodule a little bigger.
Manganese nodules grow very slowly, typically less than 0.3 inches (1 centimeter) per million years. Because they grow so slowly, some of the largest nodules can be 10 million years old. Metal nodules also grow in fresh water, usually when water flows through rocks and leaves behind deposits of metallic minerals, such as the Moqui marble of the American Southwest. Metal nodules have even been found on Mars.
Metallic nodules like these microscopic spherical grains called “blueberries” have been discovered on Mars. NASA
Metamorphic rocks that have been heated, pressured, or both can become stronger (harder to break) and heavier. Metamorphic rocks are one of three main types of rocks; the other two are igneous and sedimentary rocks. The minerals in the original rock are transformed by heat and pressure (sometimes new minerals are also grown). Metamorphic rocks become stronger when the heat and pressure cause their minerals to stick together more tightly. Additionally, a rock’s density can increase if it is subjected to pressure — packing the same amount of material into a smaller space (volume). “This makes a piece of metamorphic rock feel heavier than a piece of the same size of the original rock,” BlackEagle said.
Tiny crystals discovered in South Africa contain evidence of a sudden change in the planet’s surface 3.8 billion years ago.
These crystals, each no bigger than a grain of sand, show that around that time the Earth’s crust split and began to move – a precursor to the process known as plate tectonics.
The findings provide insight into the evolution of Earth as a planet and may help answer questions about potential links between plate tectonics and the evolution of life, said lead study author Nadja Drabon, a professor of earth and planetary sciences at Harvard University.
Today, the pieces of the rigid crust puzzle float in a sticky, hot ocean of magma in the mantle, the middle layer of the Earth. These crustal pieces rub against each other, dive under each other in so-called subduction zones, and push each other upward, creating mountains and ocean ridges, forging volcanoes, and causing earthquakes that regularly rock the planet. The sinking of tectonic plates also produces new rock in subduction zones, which interact with the atmosphere, absorbing carbon dioxide. This process makes the atmosphere more hospitable to life and keeps the climate more stable.
But it wasn’t always this way. When Earth was young and hot, during the Hadean Eon (4.6 to 4 billion years ago), the planet was first covered by an ocean of magma and then, as the planet cooled, by a solid rock surface.
Exactly when this surface cracked and its parts began to move is a subject of heated debate. Some studies estimate that plate tectonics began only 800 million years ago, while others suggest that the system is at least 2 billion years old.
Red-gold zircon crystal. Shutterstock
But because the planet is constantly recycling its crust into a mantle, there are almost no ancient rocks on the surface to help settle the debate. Before this study, rocks between 2.5 billion and 4 billion years old made up only 5% of the rocks on the surface. And no rocks survived earlier than 4 billion years.
That all changed in 2018, when Drabon and her colleagues discovered zircon crystals in a green sandstone formation in South Africa’s Barberton Greenstone Range. The team found 33 zircons that ranged in age from 4.1 billion to 3.3 billion years old.
In a new study published April 21, 2025, in the journal AGU Advances, the team analyzed different isotopes, or versions of elements with different numbers of neutrons, in ancient zircons, as well as many zircons from other times and places on Earth.
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In the isotopes, scientists have found evidence of a sudden transition to primitive plate tectonics dating back to about 3.8 billion years ago. This discovery suggests that by then, at least in one place on the planet, a simple form of subduction had begun. Whether this happened on a global scale is still undetermined, and it is likely that the “truly efficient engine of plates moving against each other” that exists today has not yet emerged.
Isotopic analysis of elements such as oxygen, niobium and uranium also showed that rocks from the surface contained water as early as 3.8 billion years ago, suggesting that the zircons were once trapped in oceanic crust buried in the primordial seafloor. And extrapolation from the earliest samples, 4.1 billion years old, suggests that the planet had a solid crust no later than 4.2 billion years ago.
This would mean that Earth’s magma sea only persisted until the Late Hadean. People previously thought that Earth was simply covered in a magma ocean until 3.6 billion years ago. The new study suggests that Earth’s molten lava ocean lasted for at most a few hundred million years before solid crust formed.
So what caused this transition? One theory is that plate tectonics simply emerged when Earth cooled enough. It’s also possible that, like a dessert spoon cracking the crisp top of a crème brûlée, massive space rocks could have slammed into Earth and fractured its crust. Another intriguing question is whether Earth’s transition to early plate tectonics helped fuel the evolution of life.
While the earliest fossil evidence of life on Earth dates back to about 3.5 billion years ago, chemical signatures of biological processes found in carbon isotope ratios are even older. Some of them can be found as far back as 3.8 billion years ago — around the same time that early plate tectonics began.
A 150,000-year history of the Earth’s magnetic field reveals secrets about the climate that existed when the first humans spread out of Africa. The data sheds light on the climate that early humans experienced when they spread out of Africa.
A tree-lined African lake has revealed data on the Earth’s magnetic field over the past 150,000 years.
A core of rock and sediment drilled from the bottom of Lake Chala, a picturesque crater lake on the border of Tanzania and Kenya, contains a record of fluctuations in the planet’s magnetic field. The rock also holds valuable information about the climate over the past 150,000 years, as modern humans migrated from Africa to the Arabian Peninsula and on to Europe and Asia.
“There are efforts to understand what conditions led humans to leave Africa and populate the rest of Eurasia,” says Anita Di Chiara, a paleomagnetist at Italy’s National Institute of Geophysics and Volcanology in Rome. But to use ancient sedimentary records to reconstruct past climates, “we need eras. We need a way to date these sediments.”
This is where changes in the Earth’s magnetic field come in. When rocks form, they record a record of the magnetic field using small magnetic crystals that align with the field at that time. Most of these rock records come from areas near the poles, where these signals are stronger.
Scientists have drilled cores from the bottom of Lake Chala (pictured above) to gain insight into the Earth’s climate over the past 150,000 years. Jacek_Sopotnicki
Scientists like Di Chiara can compare magnetic changes in rock layers from one place, like Lake Chala, to layers around the world where researchers already know how old the rocks are. They can also use layers in the rock from known events to calibrate the data. For example, the Lake Chala core includes a layer of ash from the Toba supervolcano, which erupted in Indonesia 74,000 years ago.
Lake Chala is a special place to get this data, Di Chiara said. It’s a crater lake, fed by runoff from surrounding cliffs and forests, rather than large streams or rivers. That means the layers at the bottom of the lake aren’t stirred by one-off events like floods. Instead, the layers are neatly stacked, season after season.
The researchers found six magnetic excursions — temporary, sometimes localized fluctuations in the magnetic field — in the 150,000-year-old record, they reported in April in the journal Geochemistry, Geophysics, Geosystems. One of them was an anomaly not seen anywhere else in the rock record. Such excursions may be caused by chaotic circulation in Earth’s inner core or by interactions between the solid inner core and the liquid outer core.
The magnetic excursion would probably have gone unnoticed by the ancient inhabitants of East Africa, but today the fluctuations in the magnetic field are significant. That’s because the magnetic field protects the planet from the solar wind, a stream of charged particles from the sun. A weaker field means more disruption to communications and electronic equipment from these particles.
Historical data will also help researchers predict how the magnetic field will behave.
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